1. Field of the Invention
The present invention relates to metal working, and more particularly, to a method of manufacturing mandrels for use in a piercing mill.
The invention is well adapted for application in the manufacture of water-cooled (non-detachable) mandrels incorporated in piercing mills.
In addition, the invention can also be advantageously used in the fabrication of non-cooled (detachable) mandrels employed for similar purposes.
An ever growing demand in seamless tubes, as well as high level of automation and enhanced production efficiency of installations for hot rolling of tubes, necessitate the provision of high-quality, reliable and durable tube-rolling tools.
One of the most rapidly-wearing tools is a piercing mandrel used for forming an opening in an incandescent solid blank and made to operate under conditions of high and variable pressures and temperatures, and intensive contact stresses. It should be observed that too many factors, such as production efficiency of pilger mills, quality of the inner surface of tubes being produced, and, finally, effectiveness of the pipe and tube industry, depend on the ability of mandrels to withstand the aforementioned loads. However, severe operating conditions adversely affect service life of piercing mandrels and, consequently, lead to excessively high demand in them.
To secure good surface finish in a tube, it is necessary to frequently replace the mandrel, which lowers production efficiency of rolling and raises demand for a greater number of working tools.
Constructionally, the mandrel is a spindle-shaped body streamlined in profile. The leading end of the mandrel, or its working section, is formed with a tapered nose, whereas the trailing end thereof is formed with a conical cavity adapted to serve as a fitting section of the mandrel. Provided between the working and fitting sections of the mandrel is a sizing band. In the event of rolling tubes from carbon and medium alloy steels, it is preferred to use water-cooled (non-detachable) mandrels formed with a deep, cooled cavity and a conical cavity to enable tight fitting of the mandrel on a rod. In the rolling of tubes from high-alloy difficult-to-work steels and alloys, use is made of non-cooled (detachable) mandrels formed with shallow cavity for mounting.
Water-cooled mandrels are manufactured from medium alloy steels containing from 0.1 to 0.2% C, 1% Cr, from 3 to 4% Ni, and 1% V; whereas non-cooled mandrels are fabricated from the same grades of steel, as well as from high-alloy steels and alloys with tungsten, molybdenum, cobalt, etc.
With regard to shape, the mandrel of any type is difficult to manufacture, requiring, to a lesser or greater extent, a great deal of mechanical working on separate sections of its surface. However, taking into account the fact that the mandrel is used to form a hollow in a blank in the initial stage of rolling, the required roughness of its surface, on the order of R.sub.a =300-100.mu., is ensured through casting and forging.
There are known the following methods of manufacturing mandrels (see an article by B. D. Kopysky, E. I. Semenov, L. F. Kandyba et al. in Steel magazine No. 1, pp. 55-56, 1979):
(a) subjecting a shaped casting, with or without a cavity, and effecting complete or partial mechanical treatment of this cavity;
(b) subjecting a solid or tubular blank, with or without a cavity, to forging and then effecting complete or partial mechanical treatment of the outer surface and cavity thereof.
The finished mandrel is subjected to oxidizing annealing and, if need be, the mandrel nose is spray-coated or surfaced with a high-temperature material.
Fabrication of mandrels by casting is rather efficient and effective procedure. However, due to physical specific features of the metal solidification process, the structure of metal in these mandrels is of substantially radial orientation running in perpendicular with the outer surface of the mandrel and with the sliding direction of the metal of a tubular blank, which results in an insufficient frictional engagement between the working surface of the mandrel and the blank, as well as in a low resistance to tensile tangential stress acting on the conical mounting section. Unavoidable pores and cavities, formed beneath the crust of metal, weaken the interior sections thereof and, on appearing on the surface of the mandrel in the course of its operation, they develop into defects on the interior surface of the tube. Therefore, unstable quality of the mandrels produced by casting predetermines both unstable conditions of rolling and poor quality of finished tubes. The known methods, while improving the quality of cast mandrels, namely, through additional alloying and employment of metal molds, yet substantially raise the production cost of mandrels without significantly enhancing their performance (cf. U.K. Pat. No. 1,441,052 and No. 2,176,174, and U.S. Pat. No. 3,962,897).
Today a growing number of uses are being found for forged mandrels having a compact strained structure oriented substantially along the axis of the mandrel. Such structure ensures improved mechanical properties and performance characteristics of these types of mandrels.
Irrespective of higher consumption of metal and labour, and despite the two-fold increase in the production cost, as compared with the casting method, the durability and reliability of the forged mandrels are 2 to 3 times higher, this being the decisive factor for their preferential use.
However, the forged and machined mandrels are disadvantageous from the point of view of their metallographic texture, the fibers of which are cut off during machining and thus come out onto the surface at an acute angle. Such orientation texture adversely affects wearability of the mandrel material and, consequently, shortens the service life and impairs operating stability of mandrels.
From the above it follows that prior-art methods are ineffective in that they fail to ensure the manufacture of mandrels with a requisite metal texture governing proper performance characteristics thereof.